Hesitation

When I run a 16-bit game there is a slight hesitation that gets me killed. I've tried to set the priority but that is grayed out, I've tried to run if from a command prompt using start / high <file name> but that didn't solve the problem.
I had the problem with an IBM 450 with 512-meg memory (W2000 w/SP4), and now with a new computer and keyboard the same problem (3.0 hz w/4-gig RAM and XP w/SP2).
Could it be my firewall (ZoneLab) or my anti-virus (Norton) ?
Thanks in advance for any comments or suggestions.